[ExpressionLanguage] Fixed #9606: leading spaces in expressions lead to 'Uninitialized string offset: X' warnings

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9606
| License       | MIT
| Doc PR        | 

Bug fix for #9606